I have imported an image but when I try to set the scale I am unable to change the values as my keyboard does not respond.
please take a look at the following thread:
If you are experiencing the same issue, please download Fiji or use a recent Java version.
You have to use ImageJ with Java 8 on macOS High Sierra since Java 6 is broken. ImageJ 1.51s bundled with Java 1.8.0_101 is available at https://imagej.nih.gov/ij/download.html. You will probably have to work around Apple’s path randomization “feature” when installing ImageJ on Sierra or High Sierra (https://imagej.nih.gov/ij/docs/install/osx.html#randomization).